Backwards retina: turning light to sensory signals (Introduction)

Johns Hopkins vision scientists say that their experiments, described March 12 in Proceedings of the National Academy of Sciences, show that the number of G protein molecules activated in the cascade of reactions is far fewer—involving only 10-20 of them in the rods of mice.

The new finding matters, say the scientists, because G proteins belong to a very large family of biochemical signaling pathways called G protein-coupled-receptors, which are among the most abundant signaling pathways in biology, says King-Wai Yau, Ph.D., professor of neuroscience and ophthalmology at the Johns Hopkins University School of Medicine.

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When a photon of light hits a rod in the retina, it is absorbed by a light-sensing protein, called rhodopsin, which is embedded in membranes within the cell. Rhodopsin then activates G proteins, which, in turn, activate other enzymes. It is the number of G protein molecules activated by one rhodopsin molecule that the new experiments challenge, Yau says. He notes that other scientists had speculated that the number of activated G protein molecules may be much less than the many hundred originally proposed, but the number was difficult to directly measure in intact rods.

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By using mathematical tools to analyze the electrical signal, Yue and Silverman found that the electrical signal triggered by a single G protein molecule was only one-twelfth to one-fourteenth the size of estimates of signals coming from a single rhodopsin molecule. Thus, they estimated that one rhodopsin activates approximately 10-20 G protein molecules.

Yau had previously found that, in a similar signaling cascade that facilitates the sense of smell in mice, one activated receptor molecule has a very low probability of activating one G protein molecule. By comparison, the finding that such signaling systems in vision trigger 10-20 molecules may reflect the visual system's unique need to detect light in very dim light conditions, without having to group together information from multiple rods, which would sacrifice spatial resolution.

Comment: More obvious evidence for the need for design. This process needs to be set up all at once, to function properly.